The present invention relates, in general, to intermittent mixing of two or more gas streams, and, in particular, to a new and useful apparatus for mixing relatively hot and cold flue gases with each other before they are supplied to a fabric filter during periods when the hot gas would be above the safe operating temperature of the fabric filter.
Dry scrubbing is the principal alternative to wet scrubbing for removing SO2 from flue gas released by utility boilers. Dry scrubbing is sometimes referred to as spray absorption, spray drying or semi-wet scrubbing. It involves spraying a highly atomized slurry or aqueous solution of an alkaline reagent into the hot flue gas, thereby absorbing SO2 and adiabatically cooling the flue gas. Unlike a wet scrubber installation, a dry scrubber, or spray dryer absorber, is positioned before a dust collector, such as an electrostatic precipitator or fabric filter. A fabric filter, or baghouse, collects dry particulate matter as the cooled flue gas passes through the filter material.
The publication STEAM/its generation and use 40th Ed, by Babcock & Wilcox, discloses at pages 32-9, 33-7 to 33-93 and 35-11 to 35-15, various particulars concerning fabric filters and dry scrubbers, which is incorporated herein as though fully set forth.
Application of a spray dryer absorber is limited to a flue gas volume from about 200 MWe plants on average. Larger plants require the use of several modules to treat the total flue gas flow. Baghouses fed by two or more SDAs (Spray Dry Absorbers) can be exposed to gases at temperatures that exceed the maximum allowable temperature for the bag material during periods where one SDA is taken off-line for maintenance.
Spray absorbers must be shut down periodically (e.g. bi-monthly) for maintenance on their atomizers which must be removed, or at least worked on. A spray absorber may also malfunction. During such upset periods the flue gas passing the out-of-service SDA will not be cooled sufficiently to pass to the following fabric filter structures for particle removal, since the high temperature of the gas would damage the fabric.
Such upset periods should not cause a shut down of the entire boiler or furnace or other drastic measures. The present invention seeks to solve this problem.
In order to minimize the amount of high-temperature gas entering the baghouse, the present invention proposes an adjustable thermal mixing, damper-vane apparatus and method.
During normal operation, where both flue gas streams are the same, mixing dampers in the apparatus are fully open, and do not change the direction of the flow. During an outage of one SDA, where that flue-gas temperature is much higher than normal, the dampers are moved to a configuration that promotes mixing of the two gas streams.
In the prior art, thermal protection of fabric filters has been by use of water spray, air induction or by use of a bypass flue.
U.S. patent application 2002/0079374 to DeSellem, published Jun. 27, 2002, discloses a boiler internal flue gas by-pass damper for flue gas temperature control which permits adjustment of the heat transfer effectiveness of a final bank of heat exchanger surface to control a temperature of the flue gas flowing through and exiting from the flue gas passage and conveyed to a downstream device having a minimum operating temperature.
The use of a bypass mode during startup to prevent thermal damage to fabric filters is disclosed in U.S. Pat. No. 4,322,224 to Roth. U.S. Pat. No. 4,523,931 to Miller et al. also teaches a bypass mode for gases. U.S. Pat. No. 4,372,926 to Gude et al. discloses the mixing of gases for heating the mixed gas for supply to a baghouse. U.S. Pat. No. 5,953,898 to Dryden discloses a dedicated heat exchanger upstream of a ceramic filter for controlling temperature. See U.S. Pat. No. 4,452,765 to Peterson et al., U.S. Pat. No. 5,599,508 to Martinelli and U.S. Pat. No. 5,651,948 to Myers et al. for other flue gas treatment schemes and U.S. Pat. No. 6,257,155 to Greene for a curved blade bypass structure for flue gases.
Another more common approach to provide thermal protection of fabric filters is to use bags of higher temperature rating. These bags, however, are more expensive and may not work as well. Therefore a method and apparatus that allows the use of less-expensive, lower temperature bags, while minimizing pressure drop, would be welcomed by industry.